I would also be afraid of parts getting brittle if they were frozen.

If I leave water in the drink tube for more than a few days it will get grody, but that's why I have the long brush. Bite valves clean up nicely with a brush (such as the kind that's sold as a toothpick, or a child-size toothbrush with the bristles cut down) plus a soak with a denture tablet.

The inside of the bladder is very easily cleaned - even brands with a narrow filler hole. Get a dishrag wet - somewhere between damp and sopping, depending on how much residual water is in the bladder. Shake a generous amount of baking soda into the bladder. You don't want it to dissolve completely, so that it acts as a mild abrasive. Close the filler cap/zipper and shake vigorously so that the dishrag goes all around the bladder. Change your hold a couple of times to make sure no corners get missed. Then fish out the dishrag and rinse. Shiny clean!